Advances in window technology have reduced energy consumption by affecting and improving heating, cooling and lighting. Optimal windows can accept solar gain and provide net heating. Optimization may come from coatings for windows or glass. Various types of coatings have been developed. Examples include solar control coatings that reflect across the whole spectral range to reduce glare or overheating from the sun, and low-emissivity coatings which reduce radiative heat losses often accounting for significant heat transfer through a window.
Low-emissivity coatings are well known. The coatings generally have a high reflectance in the thermal infrared (IR) and a high transmittance in the visible spectrum. Thus, they are low-emissive of thermal infrared. Some such coatings may admit solar near IR (NIR) to help heat a building, such as in a cold climate. Some such coatings may reflect the NIR back, such as in a warm climate. The low-emissivity optical properties are generally obtained by application of a material with certain intrinsic properties or alternatively, multiple materials may be combined to achieve the particular desired performance. One example of a material with relevant intrinsic properties, namely high transmittance and low-emissivity, may be doped oxides of tin or indium, wherein adjusting the dopant level can tune the wavelength cutoff between transmittance and reflectance.
Another class of materials suitable for use in providing low-emissivity includes very thin films of metals. Thin films forming infrared-reflection film are generally a conductive metal such as silver, gold or copper. Films of silver are highly reflective. The reflectance of very thin films can be suppressed by thin-film interference effects. For example, adding dielectric layers to the front and back of the metal layer reduces the reflectance of the thin film for a limited range of wavelengths. Coatings including these materials can be made highly transparent to visible radiation or light, but remain reflective in the NIR. These coatings often include one or two layers of infrared-reflection film and two or more layers of transparent dielectric film. The infrared-reflection film reduces the transmission of heat through the coating. The dielectric film is generally used to anti-reflect the infrared-reflection film and to control other properties and characteristics of the coating, such as color and durability. Such films typically have Light to Solar Gain Ratio (LSG) (visible Light Transmittance divided by the Solar Heat Gain Coefficient) ratios of >1.5. The Solar Heat Gain Coefficient is a measure which expresses the proportion of incident solar thermal radiation that is transmitted by a window. Visible Transmittance describes the amount of visible light that can pass. Each of these can be independently altered by different coatings.
Common low-emissivity coatings have one or two silver layers each sandwiched between two coats of transparent dielectric film. In order to obtain improved performance, some current systems and devices employ triple silver coatings or use a barrier as an absorbing layer. By increasing the number of silver films, the infrared reflection can be increased. Unfortunately, increasing the number of silver films also reduces visible light transmission and can negatively affect the color of the coating or decrease durability of the coating. For example, triple silver coatings have a dominant green appearance that is undesirable. Moreover, it is difficult to control the color of the coating, which can lead to color inconsistency.
Accordingly, a coating for a glass substrate is provided which provides improved performance, color control or improvement, and ease of manufacture over currently available coatings and devices.